Governments that declare war on pests like to proclaim victory. The
state of California has done that many times in its ongoing struggle with the
Mediterranean fruit fly, an invading pest that would devastate valuable
citrus, melons and other crops if allowed to spread.

If even a handful of medflies are found, officials order yard-by-yard
searches and quarantines on all fruit items. They have sprayed large swaths of
territory with the pesticide malathion and released sterile flies to disrupt
the medfly's reproductive cycle.

If no more medflies are found, they declare victory and say the infestation
has been eradicated.

Eradication efforts like California's medfly program are based on
traditional ideas about pest control: If you don't find a pest, it's not
there.

But pests have been defeating humans for centuries, often eluding the best
observation efforts. The epidemic of unwanted alien species, including
medflies and Formosan termites, has forced scientists to take a harder look at
the dynamics of how pests invade and spread, how they stay hidden and then
break out. The result is an emerging scientific discipline called invasion
biology.

What if, for example, the California program isn't really eradicating the
medfly population at all?

James Carey, an entomologist at the University of California at Davis, says
the state is wrong: Medflies are widespread in southern California and almost
impervious to episodic control efforts. The insects keep popping up in small
numbers every year or two, and Carey's studies of population and movement
patterns indicate they are there to stay.

''As hard as you try, you're not eradicating one population,'' Carey said
of the medfly program. ''It's really 10,000 to 100,000 populations, and every
little pocket can regenerate itself. The Formosan termite falls into that
category too.''

Carey's efforts are just one example of invasion biology, a science that
straddles the fields of biology, ecology, epidemiology and pest control. The
study of biological invasions has been driven by necessity.

Pest species are piling up around cities, suburbs and agricultural areas,
causing rising economic and ecological damage. They are usually impossible to
eradicate once they take hold. If Formosan termites had been discovered early
in New Orleans and their first nests destroyed, homeowners would have saved
billions of dollars over the years.

Invasions are an enduring scientific riddle. Science usually depends on
controlled experiments, but invasions can't be reproduced in a lab. So many
factors can determine whether a species survives in a new habitat that the
problem may be impossible to untangle even with a supercomputer: genes,
predator-prey relationships, climate, topography and luck.

Undeterred, a growing platoon of entomologists, botanists, marine
biologists and others has set out to examine some of the basic questions
raised by the Formosan termite debacle: Why do some species invade while
others do not? What kinds of environments and conditions are the most
vulnerable to pests? Is it possible to recognize an invader before it becomes
entrenched?

About 20 years ago, conservation biologists managing parklands started to
examine the dynamics of invasions. They had to. As alien species infiltrated
parks, they would supplant native species the scientists were trying to
protect, foreshadowing problems that would soon spring up everywhere.

Trying to understand how those species invaded and conquered the natives,
conservation biologists sensed that pests were a kind of ecological evil twin
to endangered species.

The losers in the brutal game of survival, endangered species see their
numbers dwindle as humans hunt them, take over their habitats or bring in
other species that supplant them. Pest populations multiply. Instead of being
fragile and rare, pests are tough and can live in many different places. The
fates of the two are intertwined: Invaders are a major cause of other species
going extinct worldwide.

So biologists trying to understand the principles of successful pest
behavior began by looking at something they already knew a lot about: how
species dwindle and die.

Conservation biologists and ecologists study how entire populations of
creatures behave across space and time and how their behavior follows several
rough principles:

Islands tend to have fewer species than larger land masses. Their
ecosystems, or the network of relationships among species, are simpler. In
fact, the relationship between the area of an island and the number of species
more or less follows a mathematical ratio.

Human activities have so chopped up the environment that remaining natural
areas can be described as distinct ''islands'' - forest, prairie, wetland -
surrounded by asphalted cities, suburban lawns and farms.

Species decline on these real or man-made islands. The small, isolated
populations of animals, insects and plants on them tend to thin out and die
more readily. If enough fragmented populations die, then pretty soon an entire
species may go. This phenomenon is a principal factor driving the global
process of rising extinctions and the decline in biological diversity that
scientists warn will pose major problems.

Unlike most creatures, pests thrive in a fragmented landscape. That's what
makes them costly to humans and dangerous to the environment.

They can survive in natural areas and the ''disturbed'' areas modified by
humans, though they prefer the latter. The cities and suburbs favored by the
Formosan termite are disturbed habitats where humans have assembled vast
quantities of wood that would otherwise be scattered widely.

Pests will typically colonize a human habitat, then move into natural areas
nearby, speeding the demise of the fragile, declining native species by
stealing their food, taking up space or otherwise disrupting their life
cycles.

''There are invading species that hang onto man's coattails that come to
dominate, and species that are more local that tend to decline,'' said Ted
Case, a biologist at the University of California at San Diego. Over the
years, he said, that means that thousands of local species everywhere decline,
while a relatively small number of hardier pests survive, replicating
themselves in many different places.

The edge between natural areas and developed ones is key to understanding
the invasion process. In San Diego, Argentine ants - hardy invaders that
entered the country through New Orleans in 1908 - have spread through a
patchwork of manmade and natural environments: suburban communities,
hillsides, canyons, beaches and estuaries.

That terrain contains a few relatively unspoiled areas. But the ants,
jumping off from the suburban lawns they favor, are now spoiling them.
''Urbanization provides them with moisture, shade, mulch,'' Case said. ''The
ant is less adapted to dry conditions, so it can build up in irrigated land
and then invade natural habitat.''

A team of scientists led by Case has tracked the ants' progress into two
unspoiled fragments of grassy scrub next to subdivisions. Once they invade,
Argentine ants chase off the native harvester ants, which disperse seeds for
several native plants and are the principal source of food for the endangered
coastal horned lizard. The 5-inch-long lizards have no taste for Argentine
ants. So the ant invasion has limited their diet, slowed their growth and is
slowly strangling their populations.

Scientists have a rough idea of how these broad dynamics of invasion and
extinction work. But when they zoom in on the specifics, the picture gets
blurrier. Researchers can identify some general patterns that hold across very
diverse forms of life, but are still working to explain them.

Moving species can cause things to go haywire

When a species is removed from its native environment and transplanted
somewhere else, for example, something goes haywire. An invader behaves in
ways it never would have back home.

Zebra mussels survive farther south in the United States than anywhere in
Europe. And when the dark falsemussel, a mollusk native to Louisiana, was
accidentally released into waters in the Netherlands in the 1980s, it spread
on a scale that far outstripped its modest range here.

''It's crazy over there,'' said Louisiana State University biologist Bruce
Thompson. ''It's clogging pipes, growing big reefs. It acts like this
unchecked exotic - which, over there, it is.''

In South America, Argentine ant colonies maintain discrete territories,
something that helps keep their population relatively stable. But their
cousins in the United States network their colonies. They are capable of
acting as a single unit, advancing as a single, unified wave, an effective
strategy for taking and holding territory.

Scientists offer several explanations for the phenomenon: In a native
habitat, a pest species may exist more or less in equilibrium with predators
and other creatures. But in a new place, a fast-reproducing, hardy pest
disrupts the web of relationships among other creatures, creating a kind of
biological anarchy that plays to its strengths.

Genes may also play a role. Scientists postulate that some species change
behavior when they pass through a ''genetic bottleneck.'' If only a few
invaders are present, they will inbreed. That means some common traits may be
amplified in the next generation. If those traits fit the new environment, the
behavior of the whole group may change.

The most insidious feature of pest invasions is that decades may pass
between the arrival of an invader and the time people notice it. Then, as if
following some preordained set of instructions, its growth takes off.

Scientists are still theorizing about why new pest populations display this
lag time. One explanation is simple: growing populations typically follow
exponential patterns, starting out small and then later expanding at an
accelerating rate. But a species struggling to survive at low levels in a new
habitat may also need to reach a critical mass to guarantee its long-term
survival.

Outside influences ranging from climatic changes to use of the wrong
pesticide can also unleash a pest.

Identifying a potentially serious invader before it passes its lag phase or
even before it arrives could save billions of dollars. With an eye toward just
that, some scientists are trying to devise a theory that explains and predicts
how pests behave in new environments, and helps identify the dangerous ones
before they arrive.

''The short answer is there isn't a theory, but there's going to be,'' said
University of Tennessee ecologist Daniel Simberloff, who recently brought a
team of scientists together to start work on one.

Like the whole field of invasion biology, the theory would draw on a
diverse menu of sciences in an attempt to understand difficult problems.

Population biologists such as Carey have computer models and equations that
can accurately track the spread of invaders. With more sophisticated software,
scientists can create artificial habitats and species that respond to changing
conditions, revealing some of the patterns invaders follow.

Scientists working with Simberloff are looking at features many invasions
share, such as death rates or special habitat conditions, that might give
warning signs that something is up. Because invasions are so complex, the
approach skips over questions about exactly how they occur.

''Think of it as diagnosing a patient,'' said University of Washington
biologist Peter Kareiva, who is leading the modeling effort. ''There are many
things that give an indication of how sick that person is, but we don't
necessarily want to know the process by which the disease developed.''